The present invention generally relates to filters, and in particular, to a tracking filter that tracks received signals over a relatively wide radio frequency (RF) band.
Electrical wave filters are used in communication systems to control, distinguish and/or separate certain frequency bands of an electrical signal from other signals in neighboring frequency bands. A filter is typically placed between two terminal pairs or two ports of an electrical circuit in order to modify the frequency components of a signal. Ideally, a filter will allow a certain band or bands of frequencies of a signal to pass through it with little or no attenuation. These are called the passbands of the filter. Other bands of the same signal are substantially attenuated. These bands are called the stopband or stopbands of the filter. Filters can be classified as to the range of frequencies that are passed and those that are attenuated. The more common classifications of filters are lowpass, highpass, bandpass, and bandstop filters. The present invention pertains particularly to a bandpass filter whose passband is in the radio frequency range and whose passband can be shifted in frequency electronically.
In the practical case, the band of frequencies separating a passband from its adjacent stopband or stopbands is called a transition band. The cutoff frequency of a filter defines the passband limit and usually corresponds to a frequency that is 3 dB down from the passband transmission maximum. Lowpass and highpass filters have only one cutoff frequency, while bandpass and bandstop filters have two cutoff frequencies.
If the frequency components of a signal falling within the passband of an ideal filter are applied to the input of the filter, they are xe2x80x9cpassedxe2x80x9d by the filter without attenuation to the output of the filter. Those frequency components in the stopband of the ideal filter are suppressed, and the transition band or bands have zero width. However, the amplitude of the frequency components falling within the passband frequency range of a practical RF bandpass filter may experience attenuation due to circuit absorption, reflections, radiation, and the xe2x80x9croll-offxe2x80x9d to the 3 dB cutoff frequencies at the passband edges. All of these contributions to signal attenuation result in power loss of the signal passed to the output of the filter.
A filter attenuation or insertion loss function A(xcfx89) may be defined as the decrease in power delivered to the load when a filter network is inserted between the source and the load:
A(xcfx89)=10 log10 (|Pmax|/|Pout|),
where xcfx89 is radian frequency 2xcfx80f, Pmax is maximum power available to the load, and Pout is the power delivered to the load when the filter is between the source and load. In general, the insertion loss at the minimum passband attenuation is often called xe2x80x9cflat loss.xe2x80x9d The definition of insertion loss applies at any frequency and includes the attenuation due to the filter""s selectivity.
Two major problems with tracking filters include a large value of flat loss or substantial changes to the filter""s flat loss over the tracking range or both. For example, a large value of flat loss for an RF receiver tracking filter causes the desired received RF signal to be substantially attenuated regardless of where it falls in the bandpass and adversely affects the gain distribution, the sensitivity, and the noise figure of the receiver. The change in flat loss of the RF tracking filter (and therefore the change in the passband insertion loss) over the tracking range causes another problem in the distribution and control of gain through the receiver which changes the filter""s noise figure over the tracking range. This latter problem makes certain receive bands more noisy and less desirable than other receive bands. Variation in the flat loss of the tracking filter and/or the variation in the passband insertion loss of the tracking filter over the tracking range may cause a decrease in receiver sensitivity when the passband of the tracking filter is in one position as compared to another position in the RF tracking range.
Most radio receivers are of the superheterodyne or double superheterodyne type in which the incoming RF signal is fed to a mixer and mixed with a locally-generated signal from a local oscillator. The mixer output includes an intermediate frequency (IF) signal equal to the difference between the locally-generated signal and the received RF frequency but still containing all of the original modulation. Stable high gain amplification and greater selectivity are directly attributable to the use of the intermediate frequency.
Many receivers employ xe2x80x9chigh-side injectionxe2x80x9d because the required voltage-controlled oscillator (VCO) tuning range for the synthesizer providing local oscillator frequency for the first mixer in the receiver chain is smaller (percentage-wise) for high-side injection than for low-side injection. When high-side injection is used, the problem is to design a bandpass-tracking filter of small size using practical passive element (i.e., inductors and capacitor) values. A passive filter requires no power for signal passband gain, but a control voltage may be used to change the position of the filter passband. Also, the passband of the tracking filter should have a low insertion loss and a relatively constant insertion loss as it is shifted in frequency over a relatively wide tracking range which can encompass the entire RF receive passband of the radio. In addition, the filter should have substantial attenuation on the high side of the passband.
Typically, achieving small size is associated with the simplicity of the filter (i.e., the order of the filter). On the other hand, substantial attenuation on the high side of the passband is associated with more filter complexity. Added complexity is especially a problem in achieving substantial attenuation on the high side of passband of the filter close enough to the passband to attenuate three of the most potentially troublesome and unwanted signals which may be received by the radio at fRF+(xc2xd)fif, fRF+fif, and fRF+2fif, commonly called the half-IF, the IF, and the image spurious signals, respectively. These unwanted RF signals may be produced directly (in varying degrees) in the IF band in the IF section of the receiver due to mixer nonlinearity, straight IF frequency pickup, and normal mixer action, respectively. If the RF tracking band to be covered is wide, then one or all of these unwanted signals may fall within this RF tracking band when the desired RF signal is received at the lower frequency portion of the RF tracking band. Therefore, a fixed passive RF filter with a passband corresponding to the width of the RF tracking band cannot be used because the unwanted signals would not be attenuated. However, if a narrower filter is used, then a desired RF signal received in the upper section of the RF tracking band would be attenuated. A tracking filter is needed to pass the desired RF signal received on the lower frequency section of the RF tracking band while still attenuating three spurious signals described above, one or all of which may occur within the overall RF tracking band.
One possible solution to deal with such image frequencies is to employ several passive filters, such as surface acoustic wave (SAW) filters, having different center frequencies and, using switches, select one of several filters based on the received signal frequency. The drawback with this solution is that multiple filters and switches corresponding increase the cost, complexity, and size of the radio receiver. Therefore, what is needed is a single passive bandpass filter (small size and low complexity) that can be manufactured inexpensively using realistic element values which (1) can be easily controlled to operate over a relatively wide RF tracking band, (2) has relatively low and constant insertion loss over that RF tracking band, (3) substantially attenuates undesirable frequencies on the high side of the pass band, and (4) attenuates unwanted spurious signals received inside the tracking band.
It is an object of the present invention to overcome the drawbacks and meet the needs described above by providing a single, passive bandpass filter whose passband can be shifted over a relatively wide RF tracking band while at the same time significantly attenuating undesirable spurious signals both within and outside the RF tracking band.
It is a further object of the present invention to provide a passive RF bandpass tracking filter having relatively low insertion loss of about 5.0 db or less across the RF tracking band.
It is a further object of the present invention to provide a passive RF bandpass tracking filter having a substantially constant insertion loss across the RF tracking frequency range with a variance of 1 db or less.
It is a further object of the present invention to provide a passive RF bandpass tracking filter in which both the center frequency of the filter pass band and a high side finite attenuation pole of the filter can be shifted together as the filter tracks the input RF signal.
It is a further object of the present invention to provide a passive RF tracking filter which uses standard inductor and capacitor element values and is relatively simple, low cost, and small in size.
It is yet another object of the present invention to provide a passive RF tracking filter which achieves low intermodulation (IM) distortion despite its wide RF tracking band.
The present invention provides a passive bandpass tracking filter that tracks the frequency of an RF input signal received within a radio receiver""s tracking range of RF frequencies. The bandpass filter includes parallel inductor and capacitor (LC) circuits connected in shunt between input and output terminals of the filter. A tracking control voltage is selectively applied to change the capacitance of the LC circuits in order to shift the filter""s frequency characteristic/profile as the filter tracks through the tracking band. The control signal can be selectively applied to the capacitors in the LC circuits to change the capacitance in all of the LC circuits by the same amount or by different amounts depending upon the application. The filter frequency profile includes both a center frequency of the pass band of the filter as well as a high side, finite attenuator pole of the filter which defines the frequency at maximum attenuation. Both the center frequency and finite attenuation pole shift as the filter tracks an incoming RF signal. In the example embodiment, the tracking frequency range is about 403 MHz to 470 MHz.
Half-IF, receiver IF, and image spurious signals are all attenuated. The filter substantially attenuates the image spurious signal throughout the tracking frequency range by over 50 db or more below the filter output signal level. The filter also attenuates the half-IF spurious signal throughout the tracking frequency range by well over 10 dB below the filter output signal level. The receiver IF spurious signal is attenuated about 40 dB. The bandpass tracking filter has both a low insertion loss of 5.0 dB or less and a substantially constant insertion loss variance at the RF receive frequency of about 1 dB or less over the entire tracking frequency range.
These and other objects, advantages, and features of the present invention will be described in further detail below in conjunction with the drawings.